In general, a System on Chip (SoC) used in communication systems controls power by using several methods for reducing power consumption. When the SoC is used in a Wireless Personal Area Network (WPAN), the SoC operates in an active state for a short period of time and operates in a sleep state for most periods of time to minimize power consumption.
A control method for reducing power consumption of the SoC may be implemented in a software manner. However, an operation of a Central Processing Unit (CPU) and the use of program memories may increase power consumption. Therefore, in general, it is more effective to implement the control method in a hardware manner.
There are three representative conventional methods for reducing power consumption in a hardware manner. In a first method, in order to reduce power consumption, an internal clock is off when power is off. In this method, the clock provided to all elements of the SoC can be controlled for each element by determining an active state of the SoC. Further, in this method, power consumed by a dynamic current can be reduced by turning off the clock, while power consumed by a leakage current cannot be reduced. In a WPAN-type SoC, a sleep state occupies more than 80% of chip operations,  and thus the leakage current is more important than the dynamic current. Accordingly,  the first method cannot ultimately solve the power consumption problem.
In a second method, a special register called a retention register is used. The retention register can store a present state by using data retention power when operation power is off. The retention register has an element capable of storing data by consuming a minimum leakage current by the use of retention power. However, the retention register is almost two times larger in size than a general register. In addition, since the retention register is not in a full power-off state, the leakage current leads to power consumption.
In a third method, a previous state, which is immediately before power is off, is restored through initialization in which register information of the CPU and peripherals included in the SoC is lost and then the SoC is entirely reset when power is on by performing an actual power-off.
Among the aforementioned three methods, the third method is a best method in terms of reducing power consumption. However, when power is on, it takes time to restore the previous state, which is immediately before the power is off, through reset and initialization,  and during the time, power is consumed due to an operation of the SoC.